This invention relates to radio tone signal detecting circuits in such devices as paging receivers, and more particularly it pertains to tuneable tone signal detectors having N-path filters.
A conventional tone signal detecting circuit or a selective receiver (reference is made to U.S. Pat. Nos. 3,670,242 and 3,882,466) typically consists of an active filter for sequentially passing more than one paging tone signal frequency, a detector for detecting the output of this filter, means responsive to the output of this detector for changing the center frequency and band width of the active filter, and indicator means responsive only to the final one of the paging tone signals for indicating the receiver is being paged.
However, since the center frequency, band width, voltage gain and other properties of the active filter (reference is made to, for instance, U.S. Pat. No. 3,803,429) are dependent on the resistance and capacitance of the resistor and capacitor, respectively, which constitute the filter, these values of the resistor and capacitor require high degrees of accuracy, which prove a major impediment to circuit integration and, consequently, to cost reduction.